1. Field of the Invention
The present invention relates to a variable band-pass filter circuit. More specifically, the present invention relates to a control circuit which automatically controls a variable band-pass filter constructed within an integrated circuit.
2. Description of the Prior Art
Recently, an integrated circuit used in a video recording and reproducing apparatus such as a VTR has a tendency to be constructed such that the integrated circuit incorporates a variable band-pass filter and the variable band-pass filter is automatically adjusted within the integrated circuit or from the outside.
Conventionally, as a method for automatically controlling such a kind of variable band-pass filter (hereinafter, simply called as "BPF"), a method shown in FIG. 6 or FIG. 7 is known.
In a conventional circuit 1, a first and second BPF's 2a and 2b which are to be controlled and a reference BPF 3 are used. The BPF's 2a, 2b and 3 are formed by the same circuit configuration, the same constants and the same pattern configuration in an actual integrated circuit in order to ensure strong correlation between them.
Then, to an input of the reference BPF 3, a reference signal having a predetermined frequency is applied from a reference signal source (not shown), and an output of the reference BPF 3 is applied to a phase comparator 4. In the phase comparator 4, phases of the output of the reference BPF 3 and the reference signal are compared with each other, and a control signal (a control voltage or a control current) for controlling the reference BPF 3 is outputted.
A BPF generally has a characteristic shown in FIG. 8A, wherein a phase becomes 0 degrees at a reference frequency f.sub.0, the phase varies up to minus 90 degrees when a frequency is deviated toward a higher direction, and the phase varies up to plus 90 degrees when a frequency is deviated toward a lower direction. Therefore, a level of the control signal from the phase comparator 4 represents a phase deviation, i.e., frequency deviation of the output of the reference BPF 3 and, by controlling the reference BPF 3 by means of the control signal, it is possible to adjust the reference BPF 3 such that a center frequency of the reference BPF 3 becomes coincident with a frequency f.sub.0 of the reference signal.
On the other hand, since the BPF's 2a and 2b to be controlled have a strong correlation to the reference BPF 3 as described above, by applying the control signal for controlling the reference BPF 3 to these BPF's 2a and 2b as control signals therefor, the BPF's 2a and 2b can be controlled in the same manner as that of the reference BPF 3, and as a resulting, center frequencies of the BPF's 2a and 2b become coincident with the frequency f.sub.0 of the reference signal.
In addition, in a case of the conventional circuit shown in FIG. 6, the reference BPF 3 may be replaced with an all-pass filter. In this case, the all-pass filter has a characteristic shown in FIG. 8B, for example. That is, in the all-pass filter, a phase is minus 180 degrees at a reference frequency f.sub.0, the phase varies up to minus 360 degrees when the frequency becomes high, and the phase varies up to 0 degrees when the frequency becomes low. Therefore, even if the all-pass filter is used instead the reference BPF 3, it is possible to perform an operation similar to that described above.
In another conventional circuit 1 as shown in FIG. 7, a fact that an output of a BPF is 0 degrees in phase at a reference frequency f.sub.0 and a fact that the BPF oscillates at f.sub.0 by positively feeding-back the output of the BPF to an input thereof by means of a loop having a gain of one or more are utilized. That is, in the prior art shown in FIG. 7, an amplifier 5 having a gain of one or more is connected between the output and the input of the reference BPF 3, and the output of the reference BPF 3 is positively fed-back to the input thereof through the amplifier 5. Then, an oscillation output signal of the reference BPF 3 is compared with the reference signal by the phase comparator 4 and a control signal is outputted from the phase comparator 4. As similar to the prior art in FIG. 6, the control signal is applied to the BPF's 2a and 2b to be controlled and to the reference BPF 3.
In any of the prior art shown in FIG. 6 and FIG. 7, since the BPF's 2a, 2b and 3 are formed on the same integrated circuit chip, the same show relatively strong correlation therebetween; however, when the respective BPF's 2a, 2b and 3 are to be adjusted at the same frequency f.sub.0, actually, there is a deviation or fluctuation of a frequency at a degree of about .+-.2% due to dispersion of components in the integrated circuit.
In addition, when the BPF 2a and BPF 2b are to be adjusted at different frequencies, that is, when the center frequency of the BPF 2a is adjusted to be coincident with the center frequency of the reference BPF 3 and the center frequency of the BPF 2b is to be adjusted at a half of the center frequency of the reference BPF 3, the control current from the phase comparator 4 is halved and applied to the BPF 2b. In this case, since dispersion of components of a circuit for adjusting the control current are added to the above described dispersion, the frequency may be deviated at a degree of about 5%.
Furthermore, in the prior art shown in FIG. 6, when a BPF having a high quality factor (Q) is utilized as the reference BPF in order to respond to a requirement of high accuracy, if the reference frequency f.sub.0 and the frequency of the reference BPF are deviated from each other, an output level from the reference BPF becomes small, and therefore, there is an occasion where the reference BPF cannot be controlled.